1. Field of the Invention
This invention involves Fabry-Perot cavities, and optical communication systems which include such cavities. In the inventive device part of the cavity comprises a waveguide.
2. Description of the Prior Art
The economic advantages, envisioned years ago, of transmitting information in the form of optical signals have now been realized in commercial systems. Accordingly, designs for future optical communication systems go beyond the simple transmission of information on an optical carrier, and include the processing of signals while still in optical form. Current optical transmission systems must convert the optical signal to an electronic one before processing can take place. Such processing involves standard electronic devices. In the next generation of optical communication systems the optical signal itself often will be processed without conversion to an electronic signal. Such optical processing will require optical devices which are analogous to the electronic devices used for processing electronic signals, e.g., amplifiers, modulators, filters, etc. This invention - a new type of Fabry Perot cavity - is such an optical device which can be used to process optical signals.
The Fabry-Perot cavity was invented in the late 19th century. Its operation is well understood and discussed in most of the classic texts. See, for example, Born & Wolf, Principles of Optics, MacMillan, 1959, pages 322-332. An exemplary Fabry-Perot comprises a region bounded by two plane, parallel mirrors. The structure, as an entity, transmits only certain wavelengths, for which the cavity is said to be in "resonance" - a condition obtained by appropriately adjusting the cavity parameters. At resonance the cavity transmits a series of equally spaced wavelengths. The spacing between wavelengths, called the "free spectral range" of the cavity (FSR), is a function of, among other parameters, the spacing between the mirrors.
The use of Fabry-Perot cavities to process optical signals, for example as filters, is well known. However, the application of such devices to the processing of optical signals in commercial communication systems has been hampered by, among other constraints, the lack of practical designs which had the proper characteristics, such as appropriate values of free spectral range. Nevertheless designs have been suggested that more closely meet the needs of a commercial system. For example, in Electronics Letters, Vol. 21, No. 11, pp. 504-505 (May 12, 1985), J. Stone discussed a Fabry-Perot design in which the cavity was an optical fiber waveguide with mirrored ends. The free spectral range of the resulting cavity is determined by the length of the fiber segment, and accordingly different free spectral ranges can be obtained by using fibers of different lengths. The cavity can be "tuned" over one free spectral range by changing the cavity optical length by one-half the wavelength value of the light entering the cavity. In this way the cavity can be "tuned" to resonate at, and therefore transmit, light of different wavelength values. To obtain such tuning, the cavity length can be changed by means of an exemplary piezoelectric element attached to the fiber, which, when activated, will stretch the fiber and increase the associated cavity optical length accordingly.
Among the advantages of this "fiber Fabry-Perot" is the fact that the fiber is a waveguide. This eliminates deleterious diffraction effects present in long Fabry-Perot cavities which are not waveguides. The elimination of the deleterious diffraction effects is associated with the guiding characteristics of the fiber. However, the difficulty of working with small lengths of optical fiber precludes large values of free spectral range when using fiber Fabry-Perots, and consequently limits the usefulness of the fiber Fabry-Perot design.
Large free spectral ranges can be obtained using "gap" Fabry-Perots in which the cavity is a small gap. However, because of diffraction losses longer gap cavities are less practical, and therefore the gap Fabry-Perot is not adequate for applications which require the smaller free spectral ranges otherwise associated with larger gaps. Other techniques are known to minimize diffraction losses in large gap cavities, such as the use of expanded beams. However, those techniques involve other limitations which the practitioner may desire to avoid.
It is clear that while fiber Fabry-Perots can be used where short free spectral ranges are required, and gap Fabry-Perots can be used where large free spectral range Fabry-Perots are required, there is no effective design to answer the need for mid-range Fabry-Perots.